1. Nutrients and O2 deficiency. With cell number increasing competition for substrate increases so nutrients (or oxygen for aerobic bacteria) become less available, number of dying cells arises and the viable cell population declines.
2. Toxic waste accumulation. With more nutrients absorbing cells become more metabolically active and excrete more waste products in the media which have toxic effect on population. This is another hypothesis for cells to die and decrease its number in death phase.
3. Change in pH of the media. Some metabolites or dead cells can influence the acidity of media (pH) which is a key factor of bacterial growth. As a result cells stop to divide and die declining its number.